BBC Proms in Extra High Quality on the Internet- The Tech
Rupert Brun has written a great post on the Internet Blog outlining the XHQ experiment with the Proms. He has very kindly agreed to give us on the R&D blog a more detailed technical overview of his team's work, for those of us with a liking for the techy stuff!
This post explains the signal path used to deliver the 320Kb/s AAC internet stream of Radio 3 for the final week of the BBC Proms. For background information about the experimental extra high quality feed, you may wish to read the entry on the BBC Internet Blog and to listen to the audio, visit the web page hosting the experiment here.
The Radio 3 Proms at the Royal Albert Hall, available this year in Extra High Quality. Image CC Steve Bowbrick
The signal from the microphones at the Royal Albert Hall is converted on the stage to 48ks/s 24bit audio and sent to the outside broadcast vehicle over fibre. Each microphone has appropriate equalisation and time alignment applied and the sound is mixed down to stereo for broadcast on Radio 3.
Still at 24 bit 48ks/s the stereo audio is fed over an "E1" 2Mb/s circuit to London Broadcasting House and passes through the main audio router to the Radio 3 Continuity Studio. Here it is unfortunately necessary to sample rate convert the audio to 44.1ks/s. The reasons for this are largely historic. When Radio 3 moved from analogue tape to digital production, the majority of the audio was stored on CD - either CD(R) for BBC recordings or commercial CDs. Due to limitations in faster-than-real-time sample rate conversion at the time, this in turn meant that the computer playout system used to hold audio for transmission had to operate at 44.1ks/s. so that CDs could be "ripped" into it. None the less, the playout system does work with uncompressed BWAV files rather than MP2 as was normal at the time. The same system is still in use today and it has so far not been possible to convert the system (and all the content within it) to 48ks/s. London Broadcasting House is a multi-media site and in preparation for the arrival of TV news we have set our core audio router to operate at 48ks/s. This means that for the immediate future radio works in a mixed economy of sample rates. By operating the studios and playout system at 44.1ks/s the number of conversions is minimised. For the live BBC Proms concerts there would be fewer conversions if we switched the Radio 3 continuity suite to 48ks/s but for the majority of the time this would not be the best configuration and it is not the sort of change that can be made on a regular basis - certainly not whilst the studio is on air. Once we have a new playout system (planned for 2012) it is hoped that it will be possible to operate at 48ks/s end to end, although archive content and commercial CDs will obviously need to be converted from 44.1ks/s for broadcast.
The signal passes through the continuity suite mixing desk, to allow the broadcast to be faded up at the appropriate time to become part of Radio 3's output. The mixing desk output feeds a "Transmission Router" which is used to send the studio broadcasting Radio 3 at any given time to the Radio 3 transmitters. This involves a second sample rate conversion, back to 48ks/s. The transmission router feeds a number of transmission chains such as FM, DAB and DTV (terrestrial and satellite). For Radio 3 we do not use transmission processing for any digital platform, so the feed to the DAB coders is the same as that to the digital television platforms and the internet. For other radio networks, transmission processing is used and it is matched to the platform. We use the digital television feed for the internet because we believe the processing used is the most appropriate - the bit rates and intended listening environments are similar. Ideally we would use separate transmission chains and processing for the internet but the small audience size does not yet justify the cost of this.
The audio is then fed to a small router which feeds the sound cards of our Coyopa system. Coyopa codes (with one exception) all network radio audio for the internet, including live streaming and on demand. The exception is the production of podcasts, which usually require a separate editorial version of the programme so the files for podcasts are created using the desktop production tools used to edit them.
Coyopa has two halves for resilience, each with about 60 servers. It creates audio streams for each of our network radio stations in a number of formats. For each radio station there are both national and international streams because we don't have the rights to make all of our content available outside the UK and have to give international listeners a restricted service at times. Coyopa also records all of our output according the broadcast schedule (in essence the TV Anytime Electronic Programme Guide) and uses these recordings to create the "on demand" files for programmes.
The sound cards in the servers carry out the third and final sample rate conversion to 44.1ks/s because the domestic codecs used to replay the audio in listeners' computers don't support a wide range of bit rates at 48ks/s. The sound cards are also used to provide some protection limiting and gain adjustment in order that the codecs are fed at the correct level. We feed the codecs with a peak level of -4dBFS because the codecs themselves can generate overshoots and if we fed them with a 0dBFS signal, clipping would occur. The codecs use the Fraunhofer encoder which outputs AAC-LC. The audio streams from both halves of the Coyopa system are then sent to a third party for distribution over the internet. The iPlayer just provides a link which points to the appropriate stream. The 320ks/s experimental feed uses exactly the same audio and codecs as the normal 192ks/s feed; the only difference is that the codec is set to deliver a higher bit rate.
Rupert Brun is Head of Technology for BBC Audio and Music
- Listen to Radio 3's Extra High Quality Proms audio on the Radio 3 web site during live broadcasts of The Proms until 11 September 2010. On the same page you'll find a link to a survey about the experiment. Please take a minute to complete it once you've tried the Extra High Quality experience.
- Help us spread the word about Extra Quality Audio for the Proms by tweeting about the experiment using the hashtag #PromsXHQ.
- Read Rupert's FAQ for answers to the big questions about PromsXHQ.
- Read this blog post by Radio 3 Interactive Editor Gabriel Gilson on the Radio 3 blog for some additional context.